Allow ssh-agent-based certificate authentication

[bcollet]

ncclient v0.7.0 added support to allow supplementing a public key found in an SSH agent with certificate data, if present.

This is useful for certificate authentication when the key material is only accessible through an SSH agent (for PKCS11-backed certificates for instance).

However, the current logic in PyEZ prevents using an ssh-agent while also loading a key or a certificate.

Other similar issues with this logic have been raised in the past, and forcing the ssh-agent to be disabled in certain conditions seems to cause more problems than it solves, if any:

• Do not disable SSH agent #1285
• feat: Enable using only the SSH agent if asked. #1284
• ConnectAuthError when trying to connect to SRX with SSH private key with passphrase and SSH-agent #1353

[chidanandpujar]

merged #1388